Single-Ion Behavior in New 2-D and 3-D Gadolinium 4f7 Materials: CsGd(SO4)2 and Cs[Gd(H2O)3(SO4)2]·H2O

The recent creation of 4f7 gadolinium materials has enabled vital studies of the free-ion properties of the Gd(III) cations. While the 8S ground state in a trivalent Gd compound is, in principle, isotropic, it has been demonstrated that there is a residual orbital angular momentum affected by the crystal field and structural distortion in certain systems. By exploiting the atomistic control innate to material growth, we address a fundamental question of how the isotropic nature of Gd(III) is preserved in different dimensionalities of crystal structures. To achieve this, we designed two new trivalent Gd materials possessing two structurally distinct features, a 2-D CsGd(SO4)2 and a 3-D Cs[Gd(H2O)3(SO4)2]·H2O. The tunability of the structural dimension is facilitated by O–H---O hydrogen bonds. The structural divergence between the two compounds allows us to investigate each material individually and make a comparison between them regarding their physical properties as a function of lattice dimension. Our results demonstrate that structural dimensions have a negligible effect on the single-ion behavior of the materials. Magnetization measurements for the Gd(III) complexes yielded paramagnetic states with the isotropic spin-only nature. Specific heat data suggest that there is a lack of magnetic phase transition down to T = 1.8 K, and coupled lattice vibrations in the materials are attributable to strong covalent bonding characters of the (SO4)2– and H2O ligands. This work offers a pathway for retaining the single-ion property of Gd(III) while constructing the large spin magnetic moment S = 7/2 in large-scale extended frameworks.


■ INTRODUCTION
Lanthanide (III) materials have been of particular interest in new physical phenomena owing to their capability to exhibit strong correlation physics in the presence of large spin−orbit coupling. 1−19 Among the trivalent ions of the lanthanides, gadolinium(III) is unique in that its ground state is, in principle, isotropic ( 8 S 7/2 ). The magnetic moment of the Gd 3+ cation originates from the spin S = 7/2, while the orbital angular momentum L is completely quenched. 20,21 This large spin magnetic moment of Gd 3+ materials renders them appealing systems for studies in magnetism and low-temperature magnetocalorics. 20,22−28 In addition, Gd compounds come closest to the ideal model wherein the Gd 3+ cations are completely isolated from each other and experience only a weak static field generated by the other ions. This is a prerequisite for other useful functionalities and spectroscopic investigations. 15−21 Nevertheless, structural distortion and crystal fields have been proved to influence the orbital angular momentum of some Gd 3+ materials such as Mg 2 Gd 3 Sb 3 O 14 , Gd 2 Ti 2 O 7 , and SrGd 2 O 4 , disrupting the single-ion property of these systems. [22][23][24]29 While these studies paved the way for exploring new capabilities of Gd 3+ materials, the observed magnetic anisotropy recalibrates the expectation of preserving the isotropic integrity of the 4f 7 electronic structure in extended frameworks. This is, in part, attributable to insufficient characterization of magnetism and thermodynamics of the Gd 3+ isotropy while incorporating the S = 7/2 in different dimensionalities of crystal lattice. Understanding of the degree of the isotropic character of Gd(III) is crucial in guiding design considerations of single-ion physics, qubits, and magnetocaloric coolants based on fundamental variations in the lanthanide−anion bonding as a function of lattice dimension. To address this challenge, we created two new Gd 3+ materials exhibiting distinct structural dimensionalities: 2-D CsGd(SO 4

Synthesis of C 12 H 10 N 2 OSSe (TAS)
Acetone solutions containing thiophene-2-carbonyl chloride (0.01 mol, 1.47 g, 30 mL) and KSeCN (0.01 mol, 1.44 g, 30 mL) were mixed and stirred at room temperature for 30 min. Suspended solid was removed by filtration. Acetone solution containing aniline (0.01 mol, 0.93 g) was added to the filtrate and stirred at 60°C for 1 h. TAS was isolated from the mother liquor as yellow needle crystals, washed with water and ethanol, and dried in air. Recrystallization and purification were done using 1:2 ethanol/dichloromethane mixture (yield, 1.9 g, 63.5% based on KSeCN); mp 143°C; 1  There is an excellent agreement between the PXRD and single-crystal XRD data, and no impurity was observed.

Synthesis of Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]·H 2 O
Gd(NO 3 ) 3 ·6H 2 O (2 mmol, 0.903 g) and CsCl (2 mmol, 0.337 g) were dissolved in 4 M HNO 3 (10 mL) in a 23 mL PTFE-lined autoclave, and TAS (2 mmol, 0.62 g) was added. The autoclave was heated at 200°C for 60 h and cooled slowly to 25°C at the rate of 5°C/h. A pale green solution was obtained and was left to evaporate slowly. Cs[Gd-(H 2 O) 3 (SO 4 ) 2 ]·H 2 O was isolated as block-shaped colorless crystals from the solution after 1 week. These crystals were gently washed with deionized water and dried (yield, 0.32 g, 58% based on Gd).

Single-Crystal X-ray Diffraction
Single-crystal crystallographic data of TAS (at 100 K) (Figures S1 and S2) were collected using a Bruker D8 Venture diffractometer with Mo Kα radiation (λ = 0.71073 Å) and a Photon 2 detector. Single-crystal data of CsGd(SO 4 ) 2 and Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]·H 2 O were collected on a Bruker DUO platform diffractometer equipped with a 4K CCD APEX II detector using Mo Kα radiation (λ = 0.71073 Å) and a Photon 100 detector. Data processing (SAINT) and scaling (SADABS) were performed using the Apex4 software system. The structures were solved by intrinsic phasing (SHELXT) and refined by full-matrix least-squares techniques on F 2 (SHELXL) using the SHELXTL software. 30 All atoms were refined anisotropically. Crystal structures were viewed with VESTA. 31

Powder X-ray Diffraction
Powder X-ray diffraction (PXRD) data on CsGd(SO 4 ) 2 were collected using a Rigaku Ultima IV in the 2θ range of 5−90°at a 0.2°/min scan rate. PXRD measurements on Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]·H 2 O were performed using a Bruker D2 Phaser diffractometer with a LynxEye-XE-T detector. Data were collected in the 2θ range of 5−120°at 0.64°/ min. Rietveld refinement of XRD patterns was performed using TOPAS Academic V6. 1 H and 13 C NMR spectra of TAS were obtained using a 500 MHz Bruker NMR spectrometer in CDCl 3 solution ( Figures S3 and S4).

Mass Spectroscopy
Mass spectrum of TAS was obtained from Q-TOF using matrix-assisted laser desorption/ionization (MALDI) ( Figure S5).

Thermal Analysis
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were performed using a TA SDT Q600 Instrument. Approximately 10 mg of each compound was separately placed in an alumina crucible and heated at a rate of 20°C/min from room temperature to 1000°C under flowing nitrogen (flow rate: 100 mL/min) ( Figures S8 and S10).

DC magnetization measurements on CsGd(SO 4 ) 2 and Cs[Gd-(H 2 O) 3 (SO 4 ) 2 ]·H 2 O powders
were performed with the vibrating sample magnetometer (VSM) option of the Quantum Design physical properties measurement system (PPMS). Heat capacity was measured on the single crystals using the PPMS, employing the semiadiabatic pulse technique from T = 2 to 300 K (Figures S11 and S12).

Characterization of C 12 H 10 N 2 OSSe (TAS)
The molecular structure of TAS was determined by singlecrystal X-ray diffraction and spectroscopic techniques including 1 H and 13 C NMR spectroscopy, UV−vis, IR, and mass spectroscopy. Figure S1 shows the crystal structure of TAS, and crystallographic refinement data are presented in Table S1. TAS crystallizes in the monoclinic space group P2 1 /n with four molecules in the unit cell. The molecule comprises three structural features: a thiophene ring, a phenyl ring, and an acylselenourea moiety. Further discussion on the crystal structure and spectroscopic characterization of TAS is presented in the Supporting Information ( Figures S2−S4). The chemical composition of TAS (C 12 H 10 N 2 OSSe) is consistent with that obtained by mass spectroscopy ( Figure S5). The peak at m/z = 309.94 corresponds to the mass of the parent compound (C 12 H 10 N 2 OSSe) + . We also detected fragments of the protonated species (C 12 H 11 N 2 OSSe) + at the base peak (m/z = 310.93). TAS melts incongruently at T = 143°C and then decomposes at T = 174°C as determined by the TG/DSC measurement ( Figure S8).
CsGd(SO 4 ) 2 crystallizes in the centrosymmetric orthorhombic space group Pnna. The crystal structure of the Gd material features a 2-D structure consisting of [Gd(SO 4 ) 2 ] − layers separated by the Cs + cations. Each Gd 3+ cation is bonded to eight oxygen atoms from the (SO 4 ) 2− groups, forming an eightcoordinate geometry that can be described as a distorted square antiprism. Gd−O bond distances range from 2.374(4) to 2.470(4) Å. The top basal plane of the square antiprism is rotated from the plane below by ∼29−43°, which deviates from the distortion angle for an ideal square antiprism (45°). The Gd 3+ cations form a 2-D distorted triangular sublattice with the Gd−Gd distances ranging from 5.1366(3) to 5.8952(5) Å.
The (SO 4 ) 2− anion comprises S 6+ bonded to four oxygen atoms with S−O bond lengths ranging from 1.477(4) to 1.497(4) Å. The local structure of (SO 4 ) 2− was confirmed to be the pseudo-T d point group, Γ vib = 3T 2 , as evidenced by the three peaks between 900 and 1200 cm −1 in the IR spectra ( Figure 2 confirmed to have an approximate T d point symmetry, Γ vib = 3T 2 . Each inequivalent (SO 4 ) 2− tetrahedron contributes three peaks, resulting in six peaks between 900 and 1200 cm −1 in the IR spectra ( Figure 2).

UV−Vis−NIR Spectroscopy
To probe the electronic transitions associated with the Gd 3+ cation (f 7 , S = 7/2, L = 0), we measured UV−vis−NIR reflectance spectra for CsGd(SO 4 ) 2 and Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]· H 2 O from λ = 390 to 2500 nm (hν ≈ 0.5−3.2 eV). The data are expressed as the Kubelka−Munk function F(R) versus hν ( Figure S9). 32 Both Gd compounds show no absorption band within the excitation energy window over which the measurements were performed. This behavior is consistent with other Gd 3+ systems. 33,34 The ground state of Gd 3+ is 8 S 7/2 , and the energy gap between the ground state and the next excited state ( 6 P 7/2 ) is close to 5 eV. 33,34 In addition, there is no spin-allowed transition between the ground state and the excited states. This explains the lack of electronic transition in the UV−vis−NIR data, confirming the 8 S 7/2 ground state for both Gd materials.

Thermogravimetric Analysis
The thermal behavior of CsGd(SO 4 ) 2 and Cs[Gd-(H 2 O) 3 (SO 4 ) 2 ]·H 2 O was characterized by TGA and DSC under a nitrogen atmosphere ( Figure S10). The decomposition of CsGd(SO 4 ) 2 occurs at 377°C corresponding to the loss SO 2 . The experimental weight loss (13.24%) is in excellent agreement with the calculated weight loss (13.27%). The endothermic peak in the heating curve is consistent with the decomposition of the Gd compound. In contrast to CsGd(SO 4 ) 2 , Cs[Gd-(H 2 O) 3 (SO 4 ) 2 ]·H 2 O has relatively low thermal stability. The Gd hydrate material begins to decompose at 55°C. The reduction in mass in the temperature range of 55°C − 265°C are likely attributable to the loss of 4 H 2 O molecules. The experimental weight loss (13.1%) is consistent with the calculated weight loss (13.0%).

Magnetization
To evaluate the spin state of the Gd 3+ cation in CsGd(SO 4 ) 2 and Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]·H 2 O and determine the magnetic properties of the materials, we performed temperature-dependent magnetization at μ 0 H = 9 T and field-dependent magnetization at T = 2 K (Figure 3). The magnetic susceptibility data of these materials follow the Curie−Weiss law over a wide temperature range of 30 K ≤ T ≤ 300 K (eq 1): 35 where C is the Curie constant, θ CW is the Curie−Weiss temperature, and χ 0 is the temperature-independent contribution to the susceptibility, which includes the small diamagnetic signals of the electron core and the sample holder. 36 The effective magnetic moment μ eff per Gd 3+ cation was estimated using eq 2: (2) where N A is the Avogadro number and k B is the Boltzmann constant.
The effective magnetic moments μ eff per Gd 3+ cation obtained for CsGd(SO 4 (Figure 3a,b). The Curie−Weiss temperature value was obtained to be θ CW = −1.   Additional studies such as high-frequency and high-field electron paramagnetic resonance spectroscopy, however, are encouraged to interrogate the small degree of zero-field splitting and the mixing of the 6 P high-lying excited state and the ground state.

Heat Capacity
To study the thermodynamic properties of the ground state of CsGd(SO 4 ) 2 and Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]·H 2 O, we performed heat capacity measurements at μ 0 H = 0 T, 1.8 K ≤ T ≤ 300 K. The C P /T versus T curves of these Gd materials are presented in Figure 4. As aforementioned, no magnetic phase transition was observed in the magnetization data of the Gd compounds. Thus, their total heat capacity is due to lattice excitations (phonons). The specific heat data were best described using a combination of two Debye and one Einstein modes. Equation 6 shows the fit model, and eqs 7 and 8 represent the Debye and Einstein models, respectively. 41  The C P /T data at T ≤ 6 K were analyzed using eq 9 ( Figure  4c). 42,43 = where β 3 is the thermal expansion coefficient, which describes the lattice vibration, and A is the Schottky parameter which is related to the splitting energy between nondegenerate energy levels. Electronic contribution (γ) was not included in the expression due to the insulating behavior of the compounds. The resulting β 3 Table 3. The estimated β 3 values of the two compounds are on the same order of magnitude, suggesting similar energy scale for their phonons. It is worth noting that the Schottky estimation is not ideal because only the onset of the anomaly observed down The calculated phonon was best described using a combination of two Debye models and one Einstein model (eq 6). The resulting parameters are presented in Table 2. (c) Analysis of Schottky anomaly.
to T = 1.8 K was taken into consideration. Nevertheless, the Schottky term suggests that the upturn in the specific heat data is not related to a magnetic phase transition, but rather the Schottky effect associated with the Gd 3+ paramagnetic systems.

■ CONCLUSION
Despite the isotropic nature of the 8 S 7/2 ground state in a trivalent gadolinium system, some Gd compounds show a finite orbital angular momentum which is influenced by the crystal field and structural distortion. Keeping the free-ion property of Gd(III) intact even in diverse structural constructs allows design precision and reproducibility of the spin-only nature of 4f 7 materials, yet this remains unexplored. To address this, we created two new trivalent gadolinium materials exhibiting structurally distinct features. CsGd(SO 4 ) 2 possesses a 2-D structure while Cs[Gd(H 2 O) 3 (SO 4 ) 2 ]·H 2 O has a 3-D lattice extended by O−H---O hydrogen bonds. The synthesis of these two materials demonstrates a feasible approach for investigating the single-ion behavior of Gd 3+ in dissimilar structure types. Although the lattice dimensions of the Gd complexes are different, their magnetic properties are similar and driven by the isotropic spin-only nature of the 8 S ground state of the 4f 7 electron structure. The Gd and Gd hydrate materials exhibit no magnetic phase transition down to T = 1.8 K and only a Schottky anomaly owing to the Gd 3+ paramagnetic spins. Collective lattice excitations observed from heat capacity data are likely connected to the covalent bonds of the (SO 4 ) 2− and H 2 O groups. This work illustrates a protocol for maintaining the single-ion character of the Gd 3+ cations while placing the spins in extended lattices with different dimensionalities. ■ ASSOCIATED CONTENT